ClassclassNamecondition

ClassclassNamecondition is a term used to describe the pattern of applying CSS class names conditionally across HTML and modern frontend frameworks. It emphasizes how a static class attribute in HTML relates to a framework's className property and how runtime conditions decide which classes are present.

In plain HTML, classes are fixed, but dynamic styling is achieved through template engines or scripts that assemble the class list before rendering. In React or JSX, developers assign to className rather than class, and conditional logic determines the final string. Common approaches include using a ternary expression to include an extra class, concatenating strings, or building an array of classes and joining them. Popular helper libraries, such as classnames or clsx, simplify combining truthy values into a single string.

Other frameworks offer built-in support for conditional classes. Vue uses a binding syntax like :class with object or array forms; Angular provides [ngClass] or [class.classname] bindings for similar purposes. The core idea across all these approaches is to separate styling concerns from structural markup and to adjust presentation without altering the element tree.

Advantages include more maintainable markup, easier conditional styling, and dynamic theming. Pitfalls include missing spaces, duplicate class names, and relying on framework-specific behavior that reduces portability. Best practices suggest keeping class logic small, using dedicated utilities when possible, and validating the resulting class list during development.

See also: conditional rendering, className, classNames library, clsx, v-bind:class, ngClass.